Add support for `--config-file` cli flag

[amypellegrini]

I'm currently working on a OSS project using babel-tape-runner to run my tests.

The project conventions involve having all config files inside a config/ which is possible with Babel, but not currently supported by babel-tape-runner.

I'm open to suggestions/alternative approaches as long as this allows me to keep config files inside the config/ folder, and also willing to implement this feature myself in a fork, provided I find the time and there is willingness to integrate the feature in a reasonable time frame once a PR is open.

More context, see:

• https://github.com/babel/babel/pull/6125
• https://github.com/amypellegrini/hanon-pilgrimage/issues/5
• https://babeljs.io/docs/en/options#configfile

Thanks!